1. Field of the Invention
Embodiments of the present invention relate generally to time and frequency alignment systems operating over digital communications networks and, more specifically, to a method and apparatus for finding a latency floor in packet networks.
2. Description of the Related Art
One of the more important requirements of a digital communication network is to support real-time communications applications, which typically require time or frequency alignment, or a combination of both. For example, time alignment may be used by real-time instrumentation systems gathering data at specific time intervals or operating machinery according to specific timing. Frequency alignment is typically useful to time division multiplex (TDM) systems and media streaming systems, which require fixed video or audio sample rates across multiple clients.
One approach known in the art that provides both time and frequency alignment involves computing an aligned time signal based on global positioning system (GPS) satellite timing signals, which are each held in precise alignment with a global clock reference. Using GPS signals to achieve time or frequency alignment is generally quite expensive and requires a client system to be able to receive satellite time signals from GPS satellites. In general, a more cost effective approach to time alignment is to transmit timing alignment information via a protocol that is operable within a given communications network.
In conventional TDM networks a physical layer methods implement frequency alignment throughout the network, starting with a designated master clock system. The designated master clock system delivers (frequency) timing information via bit-timing (or symbol-timing) information associated with downstream physical communication links. In normal operation each system coupled to the master clock system replicates the master clock timing information to downstream systems by replicating physical layer timing from the master clock system to each downstream system. Each system within the TDM network receives (frequency) timing information and aligns local (frequency) timing to an upstream clock reference, thereby enabling every system within the TDM network to achieve frequency alignment.
While frequency alignment within conventional TDM networks is relatively straightforward, packet-switched networks, such as networks based on the popular Ethernet industry standards, present time and frequency alignment challenges because packet-switched networks are not conventionally designed to provide precise delivery time for data or precise timing at any lower protocol levels. A key difference is that the switching and multiplexing functions are not as deterministic as circuit switching and TDM, but have a statistical aspect as well. The statistical nature of switching and multiplexing adds a different notion of quality of service. Whereas error performance is always important, the notions of delay variation and available bandwidth now come into play. For a given packet flow, such as for a circuit-emulated service, a certain minimum “bit rate” may be specified along with a measure of how much more bandwidth can be made available, depending on the level of network congestion. A Service Level Agreement (SLA) between the network provider and an end-user would specify, among other items, the guaranteed (minimum) bit rate (or equivalent) as well as the upper limit to packet delay variation and other factors that could be in jeopardy in situations of network congestion.
Furthermore, packet-switched networks typically involve multiple nodes that may store and forward data packets, potentially introducing significant transit delay variation between any two points. To generally overcome certain time alignment challenges inherent in packet-switched networks, certain time alignment protocols based on the industry standard internet protocol (IP) have been developed and deployed. One IP-based time alignment protocol is known in the art as the Network Time Protocol (NTP). NTP is used for aligning time between a master time reference and one or more clients. Precision Time Protocol (PTP) is a second IP-based time alignment protocol for aligning one or more client devices to a master time reference. PTP is defined in detail within the IEEE 1588® standard.
Persons skilled in the art will understand that NTP, PTP, and any other time alignment protocol transmitted through a packet-switched network must account for transit delay variation within the network. In fact, overall time alignment accuracy is generally determined by the ability of a system implementing time alignment to account for transit delay variation between a time reference and a clock aligning to the time reference.
Lightly loaded packet-switched networks typically present relatively low transit delay variation, allowing IP-based alignment protocols such as NTP and PTP to easily achieve excellent accuracy relative to each protocol's specification. For example, in a lightly loaded gigabit Ethernet-based network, PTP can theoretically provide alignment of better than one hundred nanoseconds. However, conventional networks typically have a wide range of bandwidth loading conditions, which leads to large transit delay variations. This transit delay variation typically leads to degradation of time alignment accuracy. Furthermore, network elements comprising the packet-switched network may process sequential packets differently, depending on prevailing congestion conditions that result from increased bandwidth loading within the network. For example, a network element may forward all packets according to one delay profile until a congestion threshold is exceeded. Once the congestion threshold is exceeded, the network element may delay high priority packets, and drop low priority packets. If congestion on the network element drops below the congestion threshold, then the network element may stop delaying high priority packets and stop dropping low priority packets.
Frequency alignment between a frequency reference (master) and a frequency client (slave) may be disrupted by abrupt changes in transit delays resulting from one or more network elements switching form normal mode to congestion mode. Because conventional frequency alignment protocols presume transit delay does not change abruptly, a conventional client device commonly interprets a change in transit delay resulting from a network element changing between normal mode and congestion mode to be a local frequency error. The conventional client device may adjust for the local frequency error, even though no adjustment is actually needed. Similarly, time alignment between a time reference (master) and a time client (slave) may also be disrupted by abrupt changes in transit delays resulting from one or more network elements switching from normal mode to congestion mode.
Thus, several factors, including network congestion and network element policies for packet processing, may contribute to greater transit delay variation in a packet-switched network. Unfortunately, transit delay variation typically reduces accuracy and reliability of time and frequency alignment systems that conventionally depend on stability of transit delay within the packet-switched network.